A fuel cell is an energy conversion apparatus that directly converts the chemical energy of a fuel to electrical energy. Therefore, there have been attempts to develop the fuel cell as a next-generation energy source due to its environmentally-friendly properties such as high-energy efficiency and low discharge of pollutants.
In the case of a moisture-free polymer electrolyte membrane fuel cell (PEMFC), it does not need a cooling apparatus since it may be operated over a wide temperature range, its sealing part is simple, it does not need a humidifier since moisture-free hydrogen is used as a fuel, and it has also been highlighted as a possible power supply in cars and households since it has an advantage of rapid operation, etc. Also, the moisture-free polymer electrolyte membrane fuel cell is a high-power fuel cell having a higher current density than other types of fuel cells, and therefore it has advantages in that it is operated over a wide temperature range, its structure is simple, and its operation and response are rapid.
A polyazole-based polybenzimidazole (®Celazole) is already known as a high-temperature fuel cell polymer electrolyte membrane. The fuel cell using the polybenzimidazole polymer electrolyte membrane generally uses moisture-free hydrogen as a fuel, and it does not need a cooling apparatus since it may be operated at 100° C. or more, especially 120° C. or more, its sealing part is simple, it does not need a humidifier, and activity of a noble metal-based catalyst present in a membrane-electrode assembly (MEA) may be enhanced. Generally, when hydrocarbon compounds such as natural gas are reformed and then used as a fuel, a catalyst is poisoned, which deteriorates a fuel cell seriously if carbon monoxide is not removed in a reformer gas workup or a purification process since a large amount of carbon monoxide is included in the reformer gas. In the case of the fuel cell using the polyazole-based polymer electrolyte membrane, a high density of carbon monoxide is allowable since the cell may be operated at a high temperature to minimize the catalyst poisoning by carbon monoxide.
The previously known polyazole-based polybenzimidazole (PBI) is generally melt-reacted with 3,3′,4,4′-tetraaminobiphenyl and isophthalic acid, or esters thereof to prepare a primary polymer, and then the resultant primary polymer is ground and polymerized into a solid state at a high temperature (400° C. or less) to prepare a polybenzimidazole polymer. In order to prepare a polybenzimidazole membrane, the polybenzimidazole, generally polymerized into a solid state at a high temperature and a high pressure using a high-pressure reactor, is dissolved in a dimethylacetamide (DMAc) solution containing a small amount of lithium chloride (LiCl), and then a membrane is prepared according to the conventional method.
German Patent No. 10109824.4 discloses a method for removing dimethylacetamide from a membrane including a high content of dimethylacetamide prepared according to the conventional method after dissolving polybenzimidazole in a dimethylacetamide (DMAc) solution containing a small amount of lithium chloride (LiCl) at a high temperature and a high pressure using a high-pressure reactor. However, it is difficult to remove the remaining dimethylacetamide using the method described in the patent, and it is also troublesome since the workup process should be conducted after preparing the membrane. If a very small amount of dimethylacetamide remains in the membrane, then the method for preparing a polybenzimidazole membrane using a dimethylacetamide solvent is problematic by itself since activity of a noble metal-based catalyst is seriously reduced due to the remaining dimethylacetamide when operating the fuel cell.
U.S. Pat. No. 5,525,436 discloses a method for preparing an ion-conductive polybenzimidazole electrolyte membrane by doping a polybenzimidazole membrane, prepared according to the conventional method, with a strong acid such as phosphoric acid or sulfuric acid, etc. If a membrane is prepared after dissolving polybenzimidazole in a dimethylacetamide solution using the conventional method in a high-pressure reactor, then the prepared membrane has a high content of dimethylacetamide, and therefore the remaining dimethylacetamide should be removed. A post strong acid-doping process described in the patent is required for ion conductivity in prepared polybenzimidazole membrane, but a dense polybenzimidazole membrane prepared by a solution pouring process is not effective. Even though the post strong acid-doping process allows ion conductivity in the polybenzimidazole membrane, the ion conductivity does not exceed 0.1 S/cm at 140° C. in the absence of moisture. Because the morphology of the electrolyte membrane, induced into a highly dense polybenzimidazole membrane prepared previously by the post strong acid doping, is not optimized between the polybenzimidazole and the strong acid, the strong acid doped at the high temperature is easily detached from the electrolyte membrane, resulting in a sudden reduction of the ion conductivity for the operation time.
U.S. Pat. No. 5,945,233 discloses a method for preparing a polybenzimidazole consistent electrolyte paste/gel. However, it is inconvenient in the process to prepare a consistent electrolyte paste/gel by further adding phosphoric acid and water to polybenzimidazole (PBI) prepared according to the conventional method, followed by stirring the resulting mixture at a high temperature so as to prepare a consistent polybenzimidazole electrolyte paste/gel as described in the patent.
U.S. Patent Publication Nos. 2004/00127588A1 and 2005/0053820A1 disclose a process for preparing a polyazole-based ion conductive polymer electrolyte membrane prepared by a process consisting of five steps. According to the patent, the process for preparing a phosphoric acid-containing polyazole-based ion conductive polymer electrolyte membrane is disclosed, including the five steps of (a) preparing a primary polymer (a precursor), (b) dissolving the primary polymer in polyphosphoric acid, (c) preparing a polyazole-based polymer from the primary polymer, (d) forming a membrane on a support, and (e) treating the formed membrane until the membrane is supported by itself, and a process for preparing an electrode coated with a polyazole-based polymer film by directly coating the electrode with the polyazole-based polymer prepared in the step (c). However, the method described in the patent has difficult steps in the process such as preparing a primary polymer (a precursor) at a high temperature, dissolving the primary polymer in polyphosphoric acid again, and then preparing a polyazole-based polymer from the primary polymer at a high temperature. In addition, the patent discloses a process for preparing an electrode coated with a polyazole-based polymer film by directly coating an electrode with polyazole-based polymer containing the polyphosphoric acid prepared in the step (c), but it has problems in that it is difficult to uniformly coat an electrode due to the very high viscosity of the polyazole-based polymer, and a workup process to hydrolyze the coated polyphosphoric acid is required for producing ion conductivity in the coated electrode.